Of the two XP is undoubtedly the cheaper option, and the most likely to let my existing software work. It may not solve my problems with e.g. monitor rotation, but getting a PC that works reliably may be more important - I'm worried that the current machine will eventually die on me.
The better PC does have a Windows XP Home COA, unfortunately I had to erase the hard disk completely. It used a recovery partition rather than a full Windows install disk, and the whole machine including the recovery partition was badly infected with viruses.
Looking on eBay, there are lots of vendors selling allegedly unused XP OEM disks with COAs pulled from upgraded PCs, around the £25 mark - I assume that this means that a company somewhere had XP pre-installed on a lot of PCs, and kept the install disks without ever using them, then went over to Vista and removed the old COAs so that they could sell the CDs.
All of them have much the same statement, which is that what they're doing is legal because they're throwing in a piece of computer hardware (probably a screw or something) with each package, and that the disks can be used on any PC. Some of them seem to have many satisfied customers, so I assume it works.
Anyone have any experience of this?